Adaptive disk spin-down method for managing the power distributed to a disk drive in a laptop computer

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BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a power distribution scheme in a portable computer and, more specifically to power distributed to a disk drive in a laptop computer.

2. Prior Art

Power consumption in an electronic device is always a significant concern. A power supply must always be designed to adequately power the device, while at the same time take into consideration other related characteristics thereof, such as its heat dissipation, physical size, weight and efficiency. These characteristics are paramount in designing or selecting an appropriate power source and become exceptionally critical when the device the power supply is to support is a self-sufficient portable unit, such as a laptop computer.

In many portable units, a self-supporting power source is used to provide the power when the unit is decoupled from its main or external power source, such as a 110 volt AC current (ordinary house current). Typically, a battery is used to provide this independent and portable power source. In some instances, the battery functions as an auxiliary power source to maintain certain critical circuits active, such as preserving data held in a volatile memory (RAM refreshment). In other instances, the battery functions as the main power source to fully power the device.

In the area of information processing, miniaturization of processing devices has permitted the portability of computing devices. One of the first such portable processing devices developed was a hand-held calculator, wherein the calculator operated from a battery power source and could easily be carried about by the user. The battery would power all of the functions of the calculator, and the user could readily transport the calculator without any attachment to an external power source. The batteries were either replaced or recharged upon being spent. The earliest calculators simply had an on/off state which could be activated on the calculator by a user. Full power was available during the on state, and the power was completely shut off during the off state. Because of the volatile nature of many early semiconductor memories in these calculators, information stored in such volatile memories was lost when the calculator was turned off. Subsequent calculators incorporated non-volatile memory to solve this problem; alternatively, standby power was provided to such a memory when the device was turned off, so that the memory retained whatever information was stored therein. More advanced schemes were devised to monitor various functions, so that power was removed from various elements when those elements were not in use. Further, a time-out scheme was later devised to put the calculator in a standby mode, such as when a key was not depressed after a certain time period, in order to preserve power. All of these features were devised primarily to extend the time period the device could operate from its internal portable power source.

When information processing technology was expanded beyond the simple calculator to encompass personal desk top computers, additional constraints were placed upon power consumption and power management control schemes. Aside from additional circuitry included within these computers that was absent from the aforesaid calculators, additional storage devices in these computers consumed large amounts of power. These memory devices included semiconductor devices, such as read-only memories (ROMs), random access memories (RAMs) (which include volatile and non-volatile memories), floppy disk drives, hard disk drives and other magnetic media. Also, additional power was required to operate the display unit in these computers which typically includes a viewing screen. Various schemes were devised to monitor and control the power distribution to these peripheral devices of a computer during on/off states.

With the advent of the portability of desk top computer systems referred to as laptops, it became desirable to provide them with fully contained, long lasting power sources. Because of their small physical size and light weight, these laptops were designed to operate only for a certain number of hours from their internal power source, i.e., typically a battery. The additional constraints imposed on desk top computers were also placed upon these laptops which contained additional circuitry, memory, viewing screens and peripheral devices attached thereto. These devices necessarily consumed additional power. In order to extend the self-sustaining time period of these laptops while keeping the battery size and weight to a minimum, a sophisticated power management scheme was essential to provide power only to those circuits and devices which required such power and to remove such power, or at least to make a given device enter a low power consumption mode, when that device was inactive. The management scheme also had to continually monitor the various circuits and devices in order that power could be applied immediately to activate such circuits and devices only when needed.

With the increasing popularity of portable laptop computers, and the industry goal to further miniaturize laptop components while enlarging memory size and laptop functions, power management of laptop system components became increasingly more important and a significant problem in the art. Table 1 below gives a listing of major laptop system components and their power consumption in a typical portable computer as measured by those skilled in the art.

TABLE 1 ______________________________________ Breakdown of power consumption by components. Manufacturer Power Percent Component & Model (watts) of Total ______________________________________ Display Compaq mono- 3.5 68% chrome lite25c Disk Drive (105 Mbytes) Maxtor 1.0 20% MLX-105 III CPU 3.3 V Intel486 0.6 12% Memory (16 Mbytes) Micron 0.024 0.5% MT4CAM4AI/B1 ______________________________________

At 68%, the display clearly dominates the system power consumption. The disk drive represents 20% of the power consumption in a portable computer. The disk thus became a clear candidate for power management because it is a device, unlike the display, with which the user does not interact directly. With proper management by the operating system, the disk may be spun up when accessed and spun down during long periods of inactivity without the user noticing much difference in performance or reliability of the system.

The recent explosion in the portable computer market enticed disk drive manufacturers to develop a special breed of disk drives especially designed for the portable environment. In addition to high shock tolerances, reduced physical volume and smaller weights, these drives consume less energy and more importantly have a new mode of operation, often called SLEEP mode.

SLEEP mode is when the disk is powered up, but the physical disk platter is not spinning. The SLEEP mode is distinguishable from IDLE mode wherein the disk is spinning but there is an absence of disk activity. ACTIVE mode is different from the SLEEP and IDLE modes in that when the disk platter is spinning, either the disk head is seeking or it is actively reading from or writing onto the disk. OFF mode is when the disk consumes no energy and performs no function except power up. Table 2 shows the power consumed by a typical disk drive as determined by those skilled in the art. Table 3 shows the transition times between disk modes and their power consumptions for a sample disk drive as determined by those skilled in the art.

TABLE 2 ______________________________________ Power consumption of the major disk modes for the Maxtor MXL-105 III. Mode Power (watts) ______________________________________ OFF 0.0 SLEEP 0.025 IDLE 1.0 ACTIVE 1.95 ______________________________________

TABLE 3 ______________________________________ Average transition times between major disk modes and their power consumptions for the Maxtor MXL-105 III. Transition Time (seconds) Power (watts) ______________________________________ POWERUP 0.5 0.025 SPINUP 2.0 3.0 SEEK 0.009 1.95 SPINDOWN 1.0 0.025 POWERDOWN 0.5 N/A ______________________________________

A very significant portion of the energy consumed by a disk drive is spent in preserving the angular momentum of the physical disk platter. A much smaller fraction is spent in powering the electrical components of the drive. By sleeping, a drive can reduce its energy consumption to near zero by allowing the disk platter to spin down to a resting state. This is aptly called spinning down the disk. This substantial energy reduction, however, is not without its costs. An access to a disk while it is sleeping incurs a delay measured in seconds as opposed to the tens of milliseconds required for an access to a spinning disk. If the disk drive is powered up and used with any frequency (multiple accesses), it will have a significant impact on the length of time the computer can operate on a single battery charge.

There is a large difference in power consumption between a disk that is spinning and one that is not. So systems try to keep the disk spinning only when they must. To get some idea of how the disk can affect battery life, power consumption of a disk on a Dell.TM. 320 SLi, a Toshiba.TM. T3300SL and a Zenith.TM. Mastersport SLe was measured by those skilled in the art. This data is shown in Table 4 below.

TABLE 4 __________________________________________________________________________ Power measurements of three typical laptop computers. CPU Speed Disk Size System Power % of Total Machine (MHz) (MBytes) Disk State Power (W) Savings (W) System Power __________________________________________________________________________ Zenith 25.0 85 Idle 10.5 1.0 9.5 Mastersport Stopped 9.5 SLe 6.5 Idle 9.2 0.9 9.8 Stopped 8.3 Toshiba 25.0 120 Idle 8.1 1.2 14.8 T3300SL Stopped 6.9 6.5 Idle 7.3 1.1 15.1 Stopped 6.2 Dell 20.0 120 Idle 4.5 0.9 20.0 320SLi Stopped 3.6 2.5 Idle 3.2 1.0 31.2 Stopped 2.2 __________________________________________________________________________

All three machines were running Mach 3.0 (UX37/MK 77). The machines are listed in the relative order of their age. At the time they were purchased, they represented the state of the art in low power notebook design. All three used the Intel.TM. SL Superset, which consists of the 386 SL CPU and the 82360 I/O controller. The Zenith.TM. and Toshiba.TM. both have a backlit LCD display, while the Dell.TM. uses a "triple super-twist nematic, reflective LCD" display. The following parameters were varied in these machines: the speed of the CPU, and the state of the disk. These parameters were controlled using hot-key bindings supplied by the system manufacturers. The CPU speed was set at the fastest and slowest speeds available; the disk was set to be either spun up or spun down.

Varying the CPU clock speed was important because the CPU can consume a large amount of power. Reducing its clock speed when there is no work to be done can significantly reduce the amount of power consumed. Mobile computers are likely to be used for highly interactive software (such as mailers, news readers, editors, etc.). So it is reasonable to expect a large amount of CPU idle time. When the CPU clock speed is reduced, a spinning disk will consume proportionally more of the total system power than when clock speed is increased.

Looking at Table 4 above, it is noted that disk densities are increasing, making it possible to carry more data. Machines are now available with even larger disks than the systems noted above. Even though disk densities have increased, the power used by the largest disks has stayed about the same, around 1W for an idle spinning disk. Next, the overall system power cost is dropping. The result is that the amount of power consumed by the disk subsystem on these notebook computers has increased from 9% to 31%. Improved recording densities make it possible to store more data on the same physical device, but they do not affect the physical mass. Drives are becoming more efficient, but cost about the same to spin up and to keep spinning. Theoretically, machines could have smaller disks, but in practice, higher recording densities are used to increase the overall capacity of the storage system instead of decreasing its power consumption. With the exception of the smallest and lightest computers, such as the Hewlett-Packard.TM. Omnibook, the trend seems to be to carry a larger disk with the same mass rather than a smaller disk with the same number of bytes.

Based on Table 4, proper power management of a disk can not only improve battery life, but can also provide a competitive advantage to one laptop over another. For instance, battery life for the Dell.TM. 320 could be improved 20 to 31%, the amount that could be saved if the disk were made inactive when not needed. Put another way, a battery that lasts 5 hours could last from 6 to 6.5 hours with proper power management. Of course, turning the disk off can result in increased access latency. After the disk is turned off, additional power will be consumed at start up, i.e. each time the disk is spun up. The system must, therefore, trade off between the power that can be saved by spinning the disk down quickly after each access and the impact on response time (including additional power consumed) from spinning it down and then up again too often.

Current laptop computers implement a number of power reduction techniques to achieve longer battery life. Most, if not all, current mobile computers use a fixed threshold to determine when to spin down the disk: if the disk has been idle for some (predetermined) amount of time, the disk is automatically spun down. The disk is spun up again upon the next access. The fixed threshold is typically on the order of many seconds or minutes to minimize the delay from on demand disk spin ups. The Hewlett-Packard.TM. Kittyhawk C3014A spins down and up again in about 3 seconds, and its manufacturer recommends spinning it down after about 5 seconds of inactivity; most other disks take several seconds for spin down/spin up and are recommended to spin down only after a period of minutes. In fact, spinning the disk for just a few seconds without accessing it can consume more power than spinning it down and up again upon the next access. Spinning down the disk more aggressively may therefore reduce the power consumption of the disk in exchange for higher latency upon the first access after the disk has been spun down.

In Table 5 below, the last entry, T.sub.d is a break even point where the cost to keep the disk spinning equals the cost in spinning it down immediately and then up again just prior to the next access. In other words, if the next access is likely to be more than T.sub.d seconds in the future, the disk should be spun down and up again prior to the next access, instead of continually spinning the disk, in order to consume less power and preserve battery life. With future knowledge of disk activity, one can spin down the disk immediately if the next access will take place more than T.sub.d seconds in the future. This will result in minimal power consumption and maximum power conservation. There are, of course, complications beyond the simple threshold; for instance, (a) a disk usually has multiple states that consume decreasing amounts of power, but from which it is increasingly costly (in time and power) to return to the active state (for example, when the disk is spinning but the disk head is parked), (b) the time of the next access is usually unpredictable, giving credence to a conservative spin-down approach, and (c) response time (spin up latency) will be adversely affected. Table 5 lists the characteristics of two disk drives for mobile computers, the Hewlett-Packard.TM. Kittyhawk C3014A and the Quantum.TM. Go-Drive 120, including values for T.sub.d.

TABLE 5 ______________________________________ Disk characteristics of the Kittyhawk C3014A and Quantum Go-Drive 120. Hewlett-Packard Kittyhawk Quantum Characteristic C3014A Go-Drive 120 _